strip heaters Fuse or breaker

jimtech67 · Jan 16, 2013

I have customer who used strip heaters to heat a metal platen.
Due to the harsh environment and process the heater fail some what frequently.

They are using Buss LPJ fuses. ( about 20 amp depending on the unit)
They want to switch to breakers.

What breakers would be appropriate for this application.
Or are break even recommended ?

Thanks

JesperMP · Jan 16, 2013

Any single phase circuit breaker will be fine.
A pure resistive load means the breaker can/should have "B" characteristic.
If you need even more monitoring, you can add current transformers with digital signal for minimum current as a feedback.

iant · Jan 16, 2013

are the heaters burning out or are the contactors/ssr's failing
as far as C/B's are concerned - use the standard 'C' curve type @ the current needed.
Note: if you are using SSR's the BUS fuse is there to protect it also.
consider upping the size of the SSR.

BryanG · Jan 16, 2013

Had a similar question from a brick maker who had problems with heat knives blowing fuses, 'should we replace the fuses with breakers?' The question should be 'how can we improve the heaters design to stop them going wrong?' We reinforced the tails and improved fixings, the failure rate went from monthly to once a year. Further small redesigns cut that even further.

jimtech67 · Jan 16, 2013

JesperMP
the load is 3 phase about 20 amps

iant
SSR are used and the heater fail not the SSRs

BryanG
I have suggested improved design to fix their issues.

TConnolly · Jan 16, 2013

A few more considerations:

1) A breaker won't operate fast enough to protect your SSR or SCR - consider some kind of supplemental fuse protection for it.

2) Make sure your breaker is rated for the available short circuit current. This is your breakers short circuit interrupt rating. Your breaker may be a 20 amp breaker, but the short circuit interrupt rating is also very important - this will be rated in kilo-Amps. When a short occurs the actual current is thousands of amps for a few milliseconds. The breaker must be able to withstand this current and still open its contacts. If you don't know how to calculate this then post back and I'll help walk you through it. (Sorry if this seems pedantic, I don't know what your experience level is)

BryanG · Jan 16, 2013

Rant warning.

Sorry to go on about it, but it one of those things that bugs the heck out of me. Don't 'suggest' they fix things, go to them with a price breakdown of production lost and costs incurred. If needed, staple it to the forehead of the guy who can't be bothered to shift his lazy backside to fix the actual problem. You can guarantee that he will be the same guy who is complaining about jobs moving overseas, you can point out that it is because of guys like him.

Rant over.

jimtech67 · Jan 16, 2013

TConnolly
no offense taken. I was an electrician (mainly machine wiring) I then started programming PLCs and writing my own code for the machined i used wire.
I tried to explain that breakers are not good idea as they are not "fast enough" but when it comes to a real technical explanation that is where I lose it.

I have told them fuses are cheaper than SSRs and easier to change.

Any input is appreciated.

Lancie1 · Jan 16, 2013

SSRs are used and the heater fails, not the SSRs.

One suggestion would be to replace failed heaters with the next larger KW heater size, but set the SSR to only put out the percentage power of max available (OLD KW / NEW KW). The SSR will control it so that the new heater only runs at partial load, but it will have heavier elements and should last longer.

Basically this is a method to replace a normal-duty heater with a "heavy-duty" heater, but not increase the maximum current in the circuit.

TConnolly · Jan 16, 2013

What is the failure mechanism of the heaters? Is it mechanical damage, chemical damage, thermal damage? Is the element failing or is the connecting wiring failing?

jimtech67 · Jan 16, 2013

its a press

the molding material infiltrates into the heater cavities and damages the heaters.
the hydraulic oil work its way in.

To stop this from happening would be extremely difficult or impossible.

Lancie1 · Jan 16, 2013

Sounds like the heater damage is from mechanical actions, not from heating and current flow. You sort of left that out at first. In which case you are mostly out of luck unless you can find a heater element with a tougher outer alloy sheath and a better lead seal. Check with Chromalox.

Protect Cartridge Heaters from External Contamination —
Contamination can occur when moisture, oil, etc. enters the sheath through the lead wires or terminal end. (The end opposite the lead wires is protected by a seal welded end disc.) Contamination frequently causes short life and dielectric failure. Special moisture resistant terminal constructions are available and hermetic seals can be supplied when severe contamination problems are present.

jimtech67 · Jan 16, 2013

yep mostly physical damage

Sorry i left that out

OkiePC · Jan 16, 2013

Be aware that SSRs tend to fail shorted so it is often a wise idea to have a shunt trip on the breaker or a separate isolation contactor so you can shut down the heater if the temp should run away (hopefully, long before product is damaged or the rising current clears fuses).

Caveman · Jan 16, 2013

Sounds maybe like you have multiple zones. Are the right thermocouples controlling
the corresponding zones? What controls these the zones? If connected wrong would cause
overheating and deformation of mold/ platen causing material to leak in.

strip heaters Fuse or breaker

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